Add missing article in "writable relationship field" in documentation

[haimingZZ]

Summary

Small docs grammar fix in docs/api-guide/relations.md.

The sentence:

The queryset argument is only ever required for writable relationship field, in which case it is used for performing the model instance lookup, that maps from the primitive user input, into a model instance.

drops the article before writable. The fix restores it to for *a writable* relationship field.

Verification

Compare with the very next paragraph in the same section, which spells out the same idea correctly with the plural form:

This behavior is now replaced with always using an explicit queryset argument for writable relational fields.

The singular form on line 462 is the only one missing the article. No code change, no behaviour change; docs-only 1-character diff.

Why this is a good first PR

• Pure docs change, 1 insertion / 1 deletion
• No tests to update, no migration
• Same shape of fix as recent merged grammar nits on these docs (e.g. PRs around allows to create/update in serializers.md and allows to choose in pagination.md)

[p-r-a-v-i-n]

For learning purpose, I wanted to know how two github users are responsible for this PR ?